Detachable coil incorporating stretch resistance

ABSTRACT

An implantable embolic device having a stretch-resistant member passing therethrough that also serves as a tether for connecting the device to a delivery system. The stretch-resistant member is attached at a proximal and distal end of the device and extends proximally to the delivery device. The proximal attachment point serves to isolate a distal, stretch resisting segment of the member from axial tension placed on a proximal, connecting section of the member. Thus, the portion of the stretch-resistant member being used to connect the embolic device to a delivery device may be placed under tension without placing tension or distorting the implant.

RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 15/081,065 filed Mar. 25, 2016 entitled DetachableCoil Incorporating Stretch Resistance, which is a continuation of andclaims priority to U.S. patent application Ser. No. 12/180,834 filedJul. 28, 2008, which claims priority to U.S. Provisional ApplicationSer. No. 60/952,520, filed Jul. 27, 2007, entitled Detachable CoilIncorporating Stretch Resistance, all of which are incorporated byreference herein in their entireties. This application also incorporatesby reference U.S. Provisional Application Ser. No. 60/604,671, filedAug. 25, 2004 entitled Thermal Detachment System For ImplantableDevices, U.S. Provisional Application Ser. No. 60/685,342 filed May 27,2005 entitled Thermal Detachment System For Implantable Devices and U.S.patent application Ser. No. 11/212,830 filed Aug. 25, 2005 entitledThermal Detachment System For Implantable Devices.

FIELD OF THE INVENTION

The present invention relates to systems and methods for deliveringimplant devices to a target site or location within the body of apatient. The present invention also relates to systems and methods fordelivering a stretch-resistant implant and a method for attaching anddetaching the implant to and from a delivery system.

BACKGROUND OF THE INVENTION

Delivery of implantable therapeutic devices by less invasive means hasbeen demonstrated to be desirable in numerous clinical situations. Forexample, vascular embolization has been used to control vascularbleeding, to occlude the blood supply to tumors, to occlude fallopiantubes, and to occlude vascular aneurysms, particularly intracranialaneurysms. In recent years, vascular embolization for the treatment ofaneurysms has received much attention. Implants used to treat aneurysmsare often convoluted or coiled lengths of wound wire and are referred toas “microcoils.” Microcoils work by filling an aneurysm causing thebloodflow through the aneurysm to slow or stop, thereby inducingthrombosis within the aneurysm.

Microcoils are extremely flexible and have very little structuralintegrity. In order to make them easier to retrieve and reposition,recent efforts have been directed to making them stretch-resistant. Forexample, a stretch-resistant embolic coil having a stretch-resistantmember passing through the interior lumen of the coil is described inU.S. Pat. No. 5,582,619 to Ken. US Patent Publication No. 2004/0034363to Wilson also discloses an embolic coil with a stretch resistant memberhaving a distal end attached near the distal end of the coil and aproximal end of the member attached to a delivery catheter.

Several different treatment modalities have been employed in the priorart for deploying implant devices. For example, numerous repositionabledetachment systems for implant devices have been described in the priorart including U.S. Pat. No. 5,895,385 to Guglielmi et al. and U.S. Pat.No. 5,108,407 to Geremia et al., the contents of which are herebyincorporated by reference. Several systems, such as those disclosed inU.S. Pat. No. 6,500,149 to Gandhi et al. and U.S. Pat. No. 4,346,712 toHanda et al., the contents of which are hereby incorporated byreference, describe the use of a heater to detach and deploy the implantdevice.

SUMMARY OF THE INVENTION

The present invention is an implant delivery and detachment system usedto position and deploy implantable devices such as coils, stents,filters, and the like within a body cavity including, but not limitedto, blood vessels, fallopian tubes, malformations such as fistula andaneurysms, heart defects (e.g. left atrial appendages and sepalopenings), and other luminal organs.

The system comprises an implant, a delivery catheter (genericallyreferred to as the pusher or delivery pusher), a detachable joint forcoupling the implant to the pusher, a heat generating apparatus(generically referred to as the heater), and a power source to applyenergy to the heater.

The present invention also includes an apparatus for delivering astretch-resistant embolic coil and a method for attaching and detachingthe coil to a delivery system. The apparatus incorporates a coil and astretch-resistant member with a distal end and a proximal end. Inaddition to providing stretch resistance, the member serves todetachably couple the implant coil to the delivery catheter.

The present invention may also be used in conjunction with the deliverymechanism disclosed in U.S. patent application Ser. No. 11/212,830 filedAug. 25, 2005 entitled “Thermal detachment system for implantingdevices,” which is incorporated by reference herein in its entirety.

In one aspect of the present invention, the implant is coupled to thepusher using a tether, string, thread, wire, filament, fiber, or thelike. Generically this is referred to as the tether. The tether may bein the form of a monofilament, rod, ribbon, hollow tube, or the like.

Many materials can be used to detachably join the implant to the pusher.One class of materials are polymers such as polyolefin, polyolefinelastomer such as those made by Dow marketed under the trade name Engageor Exxon marketed under the trade name Affinity, polyethylene, polyester(PET), polyamide (Nylon), polyurethane, polypropylene, block copolymersuch as PEBAX or Hytrel, and ethylene vinyl alcohol (EVA); or rubberymaterials such as silicone, latex, and Kraton. In some cases, thepolymer may also be cross-linked with radiation to manipulate itstensile strength and melt temperature. Another class of materials ismetals such as nickel titanium alloy (Nitinol), gold, and steel. Theselection of the material depends on the capacity of the material tostore potential energy, the melting or softening temperature, the powerused for detachment, and the body treatment site.

The tether may be joined to the implant and/or the pusher by welding,knot tying, soldering, adhesive bonding, or other means known in theart. In one embodiment where the implant is a coil, the tether may runthrough the inside lumen of the coil and be attached to the distal endof the coil. This design not only joins the implant to the pusher, butalso imparts stretch resistance to the coil without the use of asecondary stretch resistant member. In other embodiments where theimplant is a coil, stent, or filter; the tether is attached to theproximal end of the implant.

In another aspect of the present invention, the tether detachablycoupling the implant to the pusher acts as a reservoir of potentialenergy that is released during detachment. This advantageously lowersthe time and energy required to detach the implant because it allows thetether to be severed by application of heat without necessarily fullymelting the material. The stored energy also may exert a force on theimplant that pushes it away from the delivery catheter. This separationtends to make the system more reliable because it may prevent the tetherfrom re-solidifying and holding the implant after detachment.

Stored energy may be imparted in several ways. In one embodiment, aspring is disposed between the implant and pusher. The spring iscompressed when the implant is attached to the pusher by joining one endof the tether to one of either the pusher or implant, pulling the freeend of the tether until the spring is at least partially compressed,then affixing the free end of the tether to the other of the implant orthe pusher. Since both ends of the tether are restrained, potentialenergy in the form of tension on the tether (or compression in thespring) is stored within the system.

In another embodiment, one end of the tether is fixed as in the previousembodiment, and then the tether is placed in tension by pulling on thefree end of the tether with a pre-determined force or displacement. Whenthe free end of the tether is then affixed, the elongation (i.e. elasticdeformation) of the tether material itself stores energy.

In another aspect of the present invention, a heater is disposed on orwithin the pusher, typically, but not necessarily, near the distal endof the pusher. The heater may be attached to the pusher by, for example,soldering, welding, adhesive bonding, mechanical boding, or othertechniques known in the art. The heater may be in the form of a woundcoil, heat pipe, hollow tube, band, hypotube, solid bar, toroid, orsimilar shape. The heater may be made from a variety of materials suchas steel, chromium cobalt alloy, platinum, silver, gold, tantalum,tungsten, mangalin, chromium nickel alloy available from California FineWire Company under the trade name Stable Ohm, conductive polymer, or thelike. The tether is disposed in proximity to the heater. The tether maypass through the lumen of a hollow or coil-type heater or may be wrappedaround the heater. Although the tether may be disposed in direct contactwith the heater, this is not necessary. For ease of assembly, the tethermay be disposed be in proximity to, but not actually touching, theheater.

The delivery catheter or pusher is an elongate member with distal andproximal ends adapted to allow the implant to be maneuvered to thetreatment site. The pusher comprises a core mandrel and one or moreelectrical leads to supply power to the heater. The pusher may taper indimension and/or stiffness along the length, with the distal end usuallybeing more flexible than the proximal end. In one embodiment, the pusheris adapted to be telescopically disposed within a delivery conduit suchas a guide catheter or microcatheter. In another embodiment, the pushercontains an inner lumen allowing it to be maneuvered over a guide wire.In still another embodiment, the pusher can be maneuvered directly tothe treatment site without a secondary device. The pusher may have aradiopaque marking system visible with fluoroscopy that allows it to beused in conjunction with radiopaque markings on the microcatheter orother adjunctive devices.

In another aspect of the present invention, the core mandrel is in theform of a solid or hollow shaft, wire, tube, hypotube, coil, ribbon, orcombination thereof. The core mandrel may be made from plastic materialssuch as PEEK, acrylic, polyamide, polyimide, Teflon, acrylic, polyester,block copolymer such as PEBAX, or the like. The plastic member(s) may beselectively stiffened along the length with reinforcing fibers or wiresmade from metal, glass, carbon fiber, braid, coils, or the like.Alternatively, or in combination with plastic components, metallicmaterials such as stainless steel, tungsten, chromium cobalt alloy,silver, copper, gold, platinum, titanium, nickel titanium alloy(Nitinol), and the like may be used to form the core mandrel.Alternatively, or in combination with plastic and/or metalliccomponents, ceramic components such as glass, optical fiber, zirconium,or the like may be used to form the core mandrel. The core mandrel mayalso be a composite of materials.

In one embodiment, the core mandrel comprises an inner core ofradiopaque material such as platinum or tantalum and an outer coveringof kink-resistant material such as steel or chromium cobalt. Byselectively varying the thickness of the inner core, radiopaqueidentifiers can be provided on the pusher without using secondarymarkers.

In another embodiment, a core material, for example stainless steel,with desirable material properties such as kink resistance and/orcompressive strength is selectively covered (by, for example, plating,drawing, or similar methods known in the art) with a low electricalresistance material such as copper, aluminum, gold, or silver to enhanceits electrical conductivity, thus allowing the core mandrel to be usedas an electrical conductor.

In another embodiment, a core material, for example, glass or opticalfiber, with desirable properties such as compatibility with MagneticResonance Imaging (MRI), is covered with a plastic material such asPEBAX or polyimide to prevent the glass from fracturing or kinking.

In another aspect of the present invention, the heater is attached tothe pusher, and then one or more electrical conductors are attached tothe heater. In one embodiment a pair of conductive wires runssubstantially the length of the pusher and is coupled to the heater nearthe distal end of the pusher and to electrical connectors near theproximal end of the pusher. In another embodiment, one conductive wireruns the substantially the length of the pusher and the core mandrelitself is made from a conductive material or coated with a conductivematerial to act as a second electrical lead. The wire and the mandrelare coupled to the heater near the distal end and to one or moreconnectors near the proximal end of the pusher. In another embodiment, abipolar conductor is coupled to the heater and is used in conjunctionwith radiofrequency (RF) energy to power the heater. In any of theembodiments, the conductor(s) may run in parallel to the core mandrel ormay pass through the inner lumen of a substantially hollow core mandrel(for example, a hypotube).

In another aspect of the present invention, an electrical and/orthermally insulating cover or sleeve may be placed over the heater. Thesleeve may be made from insulating materials such as polyester (PET),Teflon, block copolymer, silicone, polyimide, polyamide, and the like.

In another aspect of the present invention, electrical connector(s) aredisposed near the proximal end of the pusher so that the heater can beelectrically connected to a power source through the conductors. In oneembodiment, the connectors are in the form of a plug with one or moremale or female pins. In another embodiment, the connector(s) are tubes,pins, or foil that can be connected with clip-type connectors. Inanother embodiment, the connector(s) are tubes, pins, or foil that areadapted to mate with an external power supply.

In another aspect of the present invention, the pusher connects to anexternal power source so that the heater is electrically coupled to thepower source. The power source may be from battery(s) or connected tothe electrical grid by a wall outlet. The power source supplies currentin the form of direct current (DC), alternating current (AC), modulateddirect current, or radiofrequency (RF) at either high or low frequency.The power source may be a control box that operates outside of thesterile field or may be a hand-held device adapted to operate within asterile field. The power source may be disposable, rechargeable, or maybe reusable with disposable or rechargeable battery(s).

In another aspect of the present invention, the power source maycomprise an electronic circuit that assists the user with detachment. Inone embodiment, the circuit detects detachment of the implant andprovides a signal to the user when detachment has occurred. In anotherembodiment, the circuit comprises a timer that provides a signal to theuser when a pre-set length of time has elapsed. In another embodiment,the circuit monitors the number of detachments and provides a signal orperforms an operation such as locking the system off when a pre-setnumber of detachments have been performed. In another embodiment, thecircuit comprises a feedback loop that monitors the number of attachmentattempts and increases the current, voltage, and/or detachment time inorder to increase the likelihood of a successful detachment.

In another aspect of the present invention, the construction of thesystem allows for extremely short detachment time. In one embodiment thedetachment time is less than 1 second.

In another aspect of the present invention, the construction of thesystem minimizes the surface temperature of the device duringdetachment. In one embodiment, the surface temperature at the heaterduring detachment is under 50° C. In another embodiment, the surfacetemperature at the heater during detachment is under 42° C.

In another aspect of the present invention, the implant device includesboth a stretch-resistant attribute and the ability to detachably couplethe device to the delivery catheter. This aspect of the presentinvention advantageously reduces the number of components and assemblysteps required to build a stretch-resistant implant device.

In another aspect of the present invention, tension is placed on acoupling portion of a stretch-resistant implant device while minimizingtension in a stretch-resistant portion of the same device. Thus, tensionmay be placed on the device as described in U.S. patent application Ser.No. 11/212,830 during attachment of the implant device to the deliverycatheter without placing excessive tension, which might distort theshape of the implant device (e.g., embolic coil), on the portion of theimplant device that provides stretch-resistance.

These and other aspects and features of the present invention will beappreciated upon consideration of the following drawings and detaileddescriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional side view of a first embodiment ofa detachment system according to the present invention;

FIG. 2 illustrates a cross-sectional side view of a second embodiment ofa detachment system according to the present invention;

FIG. 3A illustrates example direct signaling current according to thepresent invention;

FIG. 3B illustrates example alternating signaling current according tothe present invention;

FIG. 4 illustrates a cross-sectional side view of a third embodiment ofa detachment system according to the present invention;

FIG. 5 illustrates example temperature data of the surface of adetachment system according to the present invention;

FIG. 6 illustrates a cross-sectional side view of an electricalconnector of a detachment system according to the present invention;

FIG. 7 illustrates a cross-sectional side view of radiopaque layers of adetachment system according to the present invention; and

FIG. 8 illustrates a cross-sectional side view of a detachment systemincluding a stent according to the present invention;

FIG. 9 illustrates a side elevation view of an embodiment of an implantaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning to FIG. 1, a detachment system 100 of the present invention, andspecifically the distal portion of the detachment system 100, isillustrated. The detachment system 100 includes a pusher 102 that ispreferably flexible. The pusher 102 is configured for use in advancingan implant device 112 into and within the body of a patient and,specifically, into a target cavity site for implantation and delivery ofthe implant device 112. Potential target cavity sites include but arenot limited to blood vessels and vascular sites (e.g., aneurysms andfistula), heart openings and defects (e.g., the left atrial appendage),and other luminal organs (e.g., fallopian tubes).

A stretch-resistant tether 104 detachably couples the implant 112 to thepusher 102. In this example, the tether 104 is a plastic tube that isbonded to the pusher 102. A substantially solid cylinder could also be adesign choice for the tether 104. The stretch resistant tether 104extends at least partially through the interior lumen of an implantdevice 112.

Near the distal end of the pusher 102, a heater 106 is disposed inproximity to the stretch resistant tether 104. The heater 106 may bewrapped around the stretch resistant tether 104 such that the heater 106is exposed to or otherwise in direct contact with the blood or theenvironment, or alternatively may be insulated by a sleeve, jacket,epoxy, adhesive, or the like. The pusher 102 comprises a pair ofelectrical wires, positive electrical wire 108 and negative electricalwire 110. The wires 108 and 110 are coupled to the heater 106 by anysuitable means, such as, e.g., by welding or soldering.

The electrical wires 108, 110 are capable of being coupled to a sourceof electrical power (not shown). As illustrated the negative electricalwire 110 is coupled to the distal end of the heater 106 and the positiveelectrical wire 108 is coupled to the proximal end of the heater 106. Inanother embodiment, this configuration may be reversed, i.e., thenegative electrical wire 110 is coupled to the proximal end of theheater 106 while the positive electrical wire 108 is coupled to thedistal end of the heater 106.

Energy is applied to the heater 106 from the electrical wires 108, 110in order to sever the portion of the tether 104 in the proximity of theheater 106. It is not necessary for the heater 106 to be in directcontact with the tether 104. The heater 106 merely should be insufficient proximity to the tether 104 so that heat generated by theheater 106 causes the tether 104 to sever. As a result of activating theheater 106, the section of the stretch resistant tether 104 that isapproximately distal from the heater 106 and within the lumen of animplant device 112 is released from the pusher 102 along with theimplant device 112.

As illustrated, the implant device 112 is an embolic coil. An emboliccoil suitable for use as the implant device 112 may comprise a suitablelength of wire formed into a helical microcoil. The coil may be formedfrom a biocompatible material including platinum, rhodium, palladium,rhenium, tungsten, gold, silver, tantalum, and various alloys of thesemetals, as well as various surgical grade stainless steels. Specificmaterials include the platinum/tungsten alloy known as Platinum 479 (92%Pt, 8% W, available from Sigmund Cohn, of Mount Vernon, N.Y.) andnickel/titanium alloys (such as the nickel/titanium alloy known asNitinol).

Another material that may be advantageous for forming the coil is abimetallic wire comprising a highly elastic metal with a highlyradiopaque metal. Such a bimetallic wire would also be resistant topermanent deformation. An example of such a bimetallic wire is a productcomprising a Nitinol outer layer and an inner core of pure referencegrade platinum, available from Sigmund Cohn, of Mount Vernon, N.Y., andAnomet Products, of Shrewsbury, Mass.

Commonly-assigned U.S. Pat. No. 6,605,101 provides a further descriptionof embolic coils suitable for use as the implant device 112, includingcoils with primary and secondary configurations wherein the secondaryconfiguration minimizes the degree of undesired compaction of the coilafter deployment. The disclosure of U.S. Pat. No. 6,605,101 is fullyincorporated herein by reference. Furthermore, the implant device 112may optionally be coated or covered with a hydrogel or a bioactivecoating known in the art.

The coil-type implant device 112 resists unwinding because the stretchresistant tether 104 that extends through the lumen of the implantdevice 112 requires substantially more force to plastically deform thanthe implant device 112 itself. The stretch resistant tether 104therefore assists in preventing the implant device 112 from unwinding insituations in which the implant device 112 would otherwise unwind.

During assembly, potential energy may be stored within the device tofacilitate detachment. In one embodiment, an optional spring 116 isplaced between the heater 106 and the implant device 112. The spring iscompressed during assembly and the distal end of the tether 104 may betied or coupled to the distal end of the implant device 112, or may bemelted or otherwise formed into an atraumatic distal end 114.

In one embodiment, the stretch resistant tether 104 is made from amaterial such as a polyolefin elastomer, polyethylene, or polypropylene.One end of the tether 104 is attached to the pusher 102 and the free endof the tether 104 is pulled through the implant 112 with the proximalend of the implant 112 flush to either the heater 106 (if no spring 116is present) or to the compressed spring 116. A pre-set force ordisplacement is used to pre-tension the tether 104, thus storing energyin an axial orientation (i.e. co-linear or parallel to the long axis ofthe pusher 102) within the tether 104. The force or displacement dependson the tether material properties, the length of the tether 104 (whichitself depends on the tether's attachment point on the pusher and thelength of the implant). Generally, the force is below the elastic limitof the tether material, but sufficient to cause the tether to severquickly when heat is applied. In one preferred embodiment wherein theimplant to be deployed is a cerebral coil, the tether has a diameterwithin the range of approximately 0.001 to 0.007 inches. Of course thesize of the tether can be changed to accommodate different types andsizes of other implants as necessary.

Turning to FIG. 2, another embodiment of a detachment system of thepresent invention, detachment system 200, is illustrated. Detachmentsystem 200 shares several common elements with detachment system 100.For example, the same devices usable as the implant device 112 withdetachment system 100 are also usable as the implant device 112 withdetachment system 200. These include, e.g., various embolic microcoilsand coils. The implant device 112 has been previously described withrespect to detachment system 100. As with the implant device 112, thesame identification numbers are used to identify otherelements/components of detachment system 100 that may correspond toelements/components of detachment system 200. Reference is made to thedescription of these elements in the description of detachment system100 as that description also applies to these common elements indetachment system 200.

With detachment system 200, an interior heating element 206 is used toseparate a section of a stretch resistant tube 104 and an associatedimplant device 112 from the detachment system 200. Detachment system 200includes a delivery pusher 202 that incorporates a core mandrel 218. Thedetachment system 200 further includes a positive electrical wire 208and a negative electrical wire 210 that extend through the lumen of thedelivery pusher 202.

To form the internal heating element 206, the positive electrical wire208 and the negative electrical wire 210 may be coupled to the coremandrel 218 of the delivery pusher 202. Preferably, the electrical wires208, 210 are coupled to a distal portion of the core mandrel 218.

In one embodiment, the positive electrical wire 208 is coupled to afirst distal location on the core wire 218, and the negative electricalwire 210 is coupled to a second distal location on the core mandrel 218,with the second distal location being proximal to the first distallocation. In another embodiment, the configuration is reversed, i.e.,the positive electrical wire 208 is coupled to the second distallocation and the negative electrical wire 210 is coupled to the firstdistal location on the core mandrel 218. When the positive electricalwire 208 and the negative electrical wire 210 are coupled to the distalportion of the core mandrel 218, the distal portion of the core mandrel218 along with the electrical wires 208, 210 forms a circuit that is theinterior heating element 206.

The heater 206 increases in temperature when a current is applied from apower source (not shown) that is coupled to the positive electrical wire208 and the negative electrical wire 210. As a result, the portion ofthe stretch resistant tether 104 in proximity to the heater 206 seversand is detached from the detachment system 200 along with the implantdevice 112.

Because the heater 206 is located within the lumen of the tube-shapedtether 104, the heater 206 is insulated from the body of the patient. Asa result, the possibility of inadvertent damage to the surrounding bodytissue due to the heating of the heater 206 may be reduced.

In one embodiment of the detachment system 200, the proximal end of thestretch resistant tether 104 (or the distal end of a larger tube (notshown) coupled to the proximal end of the stretch resistant tether 104)may be flared in order to address size constraints and facilitate theassembly of the detachment system 200.

In a similar manner as with detachment system 100, energy may be storedwithin the system with, for example, an optional compressive spring 116or by pre-tensioning the tether 104 during assembly as previouslydescribed. When present, the release of potential energy stored in thesystem operates to apply additional pressure to separate the implantdevice 112, and the portion of the stretch resistant tether 104 to whichthe implant device 112 is coupled, away from the heater 206 when theimplant device 112 is deployed. This advantageously lowers the requireddetachment time and temperature by causing the tether 104 to sever andbreak.

As with detachment system 100, the distal end of the stretch resistanttether 104 of detachment system 200 may be tied or coupled to the distalend of the implant device 112, or may be melted or otherwise formed intoan atraumatic distal end 114.

FIG. 4 illustrates another preferred embodiment of a detachment system300. In many respects, the detachment system 300 is similar to thedetachment system 200 shown in FIG. 2 and detachment system 100 shown inFIG. 1. For example, the detachment system 300 includes a deliverypusher 301 containing a heater 306 that detaches an implant device 302.Detachment system 300 also utilizes a tether 310 to couple the implantdevice 302 to the delivery pusher 301.

In the cross-sectional view of FIG. 4, a distal end of the deliverypusher 301 is seen to have a coil-shaped heater 306 that is electricallycoupled to electrical wires 308 and 309. These wires 308, 309 aredisposed within the delivery pusher 301, exiting at a proximal end ofthe delivery pusher 301 and coupling to a power supply (not shown). Thetether 310 is disposed in proximity to the heater 306, having a proximalend fixed within the delivery pusher 301 and a distal end coupled to theimplant device 302. As current is applied through wires 308 and 309, theheater 306 increases in temperature until the tether 310 breaks,releasing the implant device 302.

To reduce the transfer of heat from the heater 306 to the surroundingtissue of the patient and to provide electrical insulation, aninsulating cover 304 is included around at least the distal end of theouter surface of the delivery pusher 301. As the thickness of the cover304 increases, the thermal insulating properties also increase. However,increased thickness also brings increased stiffness and a greaterdiameter to the delivery pusher 301 that could increase the difficultyof performing a delivery procedure. Thus, the cover 304 is designed witha thickness that provides sufficient thermal insulating propertieswithout overly increasing its stiffness.

To enhance attachment of the tether 310 to the implant device 302, theimplant device 302 may include a collar member 322 welded to the implantdevice 302 at weld 318 and sized to fit within the outer reinforcedcircumference 312 of the delivery pusher 301. The tether 310 ties aroundthe proximal end of the implant device 302 to form knot 316. Furtherreinforcement is provided by an adhesive 314 that is disposed around theknot 316 to prevent untying or otherwise unwanted decoupling.

In a similar manner as with detachment systems 100 and 200, energy maybe stored within the system with, for example, an optional compressivespring (similar to compressive spring 116 in FIG. 1 but not shown inFIG. 4) or by axially pre-tensioning the tether 104 during assembly. Inthis embodiment, one end of the tether 310 is attached near the proximalend of the implant device 302 as previously described. The free end ofthe tether 310 is threaded through a distal portion of the deliverypusher 301 until it reaches an exit point (not shown) of the deliverypusher 301. Tension is applied to the tether 310 in order to storeenergy in the form of elastic deformation within the tether material by,for example, placing a pre-determined force on the free end of thetether 310 or moving the taut tether 310 a pre-determined displacement.The free end of the tether 310 is then joined to the delivery pusher 301by, for example, tying a knot, applying adhesive, or similar methodsknown in the art.

When present, the release of potential energy stored in the systemoperates to apply additional pressure to separate the implant device302, and the portion of the tether 310 to which the implant device 302is coupled, away from the heater 306 when the implant device 302 isdeployed. This advantageously lowers the required detachment time andtemperature by causing the tether 310 to sever and break.

The present invention also provides for methods of using detachmentsystems such as detachment systems 100, 200, or 300. The followingexample relates to the use of detachment system 100, 200, or 300 foroccluding cerebral aneurysms. It will, however, be appreciated thatmodifying the dimensions of the detachment system 100, 200, or 300 andthe component parts thereof and/or modifying the implant device 112, 302configuration will allow the detachment system 100, 200, or 300 to beused to treat a variety of other malformations within a body.

With this particular example, the delivery pusher 102, 202, or 301 ofthe detachment system 100, 200, or 300 may be approximately 0.010 inchesto 0.030 inches in diameter. The tether 104, 310 that is coupled nearthe distal end of the delivery pusher 102, 202, or 301 and is coupledthe implant device 112, 302 may be 0.0002 inches to 0.020 inches indiameter. The implant device 112, 302; which may be a coil, may beapproximately 0.005 inches to 0.020 inches in diameter and may be woundfrom 0.0005 inch to 0.005 inch wire.

If potential energy is stored within the detachment system 100, 200, or300, the force used to separate the implant device 112, 302 typicallyranges up to 250 grams.

The delivery pusher 102, 202, or 301 may comprise a core mandrel 218 andat least one electrically conductive wire 108, 110, 208, 210, 308, or309. The core mandrel 218 may be used as an electrical conductor, or apair of conductive wires may be used, or a bipolar wire may be used aspreviously described.

Although the detachment systems 100, 200, and 300 have been illustratedas delivering a coil, other implant devices are contemplated in thepresent invention. For example, FIG. 8 illustrates the detachment system300 as previously described in FIG. 4 having an implant that is a stent390. This stent 390 could similarly be detached by a similar method aspreviously described in regards to the detachment systems 100, 200, and300. In a further example, the detachment systems 100, 200, or 300 maybe used to deliver a filter, mesh, scaffolding or other medical implantsuitable for delivery within a patient.

FIG. 7 presents an embodiment of a delivery pusher 350, which could beused in any of the embodiments as delivery pusher 102, 202, or 301,which includes radiopaque materials to communicate the position of thedelivery pusher 350 to the user. Specifically, the radiopaque markermaterial is integrated into the delivery pusher 350 and varied inthickness at a desired location, facilitating easier and more precisemanufacturing of the final delivery pusher 350.

Prior delivery pusher designs, such as those seen in U.S. Pat. No.5,895,385 to Guglielmi, rely on high-density material such as gold,tantalum, tungsten, or platinum in the form of an annular band or coil.The radiopaque marker is then bonded to other, less dense materials,such as stainless steel, to differentiate the radiopaque section. Sincethe radiopaque marker is a separate element placed at a specifieddistance (often about 3 cm) from the tip of the delivery pusher, theplacement must be exact or the distal tip of the delivery pusher 350 canresult in damage to the aneurysm or other complications. For example,the delivery pusher 350 may be overextended from the microcatheter topuncture an aneurysm. Additionally, the manufacturing process to make aprior delivery pusher can be difficult and expensive, especially whenbonding dissimilar materials.

The radiopaque system of the present invention overcomes thesedisadvantages by integrating a first radiopaque material into most ofthe delivery pusher 350 while varying the thickness of a secondradiopaque material, thus eliminating the need to bond multiple sectionstogether. As seen in FIG. 7, the delivery pusher 350 comprises a coremandrel 354 (i.e. the first radiopaque material), preferably made fromradiopaque material such as tungsten, tantalum, platinum, or gold (asopposed to the mostly radiolucent materials of the prior art designssuch as steel, Nitinol, and Elgiloy).

The delivery pusher 350 also includes a second, outer layer 352, havinga different radiopaque level. Preferably, outer layer 352 is composed ofa material having a lower radiopaque value than the core mandrel 354,such as Elgiloy, Nitinol, or stainless steel (commercially availablefrom Fort Wayne Metals under the trade name DFT). In this respect, boththe core mandrel 354 and the outer layer 352 are visible anddistinguishable from each other under fluoroscopy. The outer layer 352varies in thickness along the length of the delivery pusher 350 toprovide increased flexibility and differentiation in radio-density. Thusthe thicker regions of the outer layer 352 are more apparent to the userthan the thinner regions under fluoroscopy.

The transitions in thickness of the outer layer 352 can be preciselycreated at desired locations with automated processes such as grinding,drawing, or forging. Such automated processes eliminate the need forhand measuring and placement of markers and further eliminates the needto bond a separate marker element to other radiolucent sections, thusreducing the manufacturing cost and complexity of the system.

In the present embodiment, the delivery pusher 350 includes three mainindicator regions of the outer layer 352. A proximal region 356 is thelongest of the three at 137 cm, while a middle region 358 is 10 cm and adistal region 360 is 3 cm. The length of each region can be determinedbased on the use of the delivery pusher 350. For example, the 3 cmdistal region 360 may be used during a coil implant procedure, as knownin the art, allowing the user to align the proximal edge of the distalregion 360 with a radiopaque marker on the microcatheter within whichthe delivery pusher 350 is positioned. The diameter of each of theregions depends on the application and size of the implant. For atypical cerebral aneurysm application for example, the proximal region356 may typically measure 0.005-0.015 inches, the middle region 358 maytypically measure 0.001-0.008 inches, while the distal region 360 maytypically measure 0.0005-0.010 inches. The core mandrel 354 willtypically comprise between about 10-80% of the total diameter of thedelivery pusher 350 at any point.

Alternately, the delivery pusher 350 may include any number of differentregions greater than or less than the three shown in FIG. 7.Additionally, the radiopaque material of the core mandrel 354 may onlyextend partially through the delivery pusher 350. For example, theradiopaque material could extend from the proximal end of the coremandrel 354 to three centimeters from the distal end of the deliverypusher 350, providing yet another predetermined position marker visibleunder fluoroscopy.

In this respect, the regions 356, 358, and 360 of delivery pusher 350provide a more precise radiopaque marking system that is easilymanufactured, yet is readily apparent under fluoroscopy. Further, theincreased precision of the markers may decrease complications relatingto improper positioning of the delivery pusher during a procedure.

In operation, the microcatheter is positioned within a patient so that adistal end of the microcatheter is near a target area or lumen. Thedelivery pusher 350 is inserted into the proximal end of themicrocatheter and the core mandrel 354 and outer layer 352 are viewedunder fluoroscopy. The user aligns a radiopaque marker on themicrocatheter with the beginning of the distal region 360, whichcommunicates the location of the implant 112, 302 relative to the tip ofthe microcatheter.

In some situations, for example, small aneurysms where there may be anelevated risk of vessel damage from the stiffness of the delivery pusher350, the user may position the proximal end of the implant slightlywithin the distal end of the microcatheter during detachment. The userthen may push the proximal end of the implant 112, 302 out of themicrocatheter with the next coil, an adjunctive device such asguidewire, or the delivery pusher 102, 202, 301, or 350. In anotherembodiment, the user may use the radiopaque marking system to locate thedistal end of the delivery pusher outside the distal end of themicrocatheter.

Once the implant device 112, 302 of the detachment system 100, 200, or300 is placed in or around the target site, the operator may repeatedlyreposition the implant device 112, 302 as necessary or desired.

When detachment of the implant device 112, 302 at the target site isdesired, the operator applies energy to the heater 106, 206, or 306 byway of the electrical wires 108, 110, 208, 210, 308, or 309. Theelectrical power source for the energy may be any suitable source, suchas, e.g., a wall outlet, a capacitor, a battery, and the like. For oneaspect of this method, electricity with a potential of approximately 1volt to 100 volts is used to generate a current of 1 milliamp to 5000milliamps, depending on the resistance of the detachment system 100,200, or 300.

One embodiment of a connector system 400 that can be used toelectrically couple the detachment system 100, 200, or 300 to the powersource is shown in FIG. 6. The connector system 400 includes anelectrically conductive core mandrel 412 having a proximal endsurrounded by an insulating layer 404. Preferably the insulating layer404 is an insulating sleeve such as a plastic shrink tube of polyolefin,PET, Nylon, PEEK, Teflon, or polyimide. The insulating layer 404 mayalso be a coating such as polyurethane, silicone, Teflon, paralyene. Anelectrically conductive band 406 is disposed on top of the insulatinglayer 404 and secured in place by molding bands 414, adhesive, or epoxy.Thus, the core mandrel 412 and the conductive band 406 are electricallyinsulated from each other. The conductive band 406 is preferablycomposed of any electrically conductive material, such as silver, gold,platinum, steel, copper, conductive polymer, conductive adhesive, orsimilar materials, and can be a band, coil, or foil. Gold is especiallypreferred as the conductive material of the conductive band 406 becauseof the ability of gold to be drawn into a thin wall and its readyavailability. The core mandrel 412 has been previously described and maybe plated with, for example, gold, silver, copper, or aluminum toenhance its electrical conductivity.

The connector system 400 also includes two electrical wires 408 and 410which connect to the conductive band 406 and core member 412,respectively, and to a heating element at the distal end of a deliverysystem such as those described in FIGS. 1, 2, and 4 (not shown in FIG.6). These wires 408 and 410 are preferably connected by soldering,brazing, welding, laser bonding, or conductive adhesive, or similartechniques.

Once the user is ready to release the implant 112, 302 within thepatient, a first electrical clip or connector from a power source isconnected to a non-insulated section 402 of the core mandrel 412 and asecond electrical clip or connector from the power source is connectedto the conductive band 406. Electrical power is applied to the first andsecond electrical clips, forming an electrical circuit within thedetachment system 100, 200, or 300, causing the heater 106, 206, or 306to increase in temperature and sever the tether 104, 310.

Once the detachment system 100, 200, or 300 is connected to the powersource the user may apply a voltage or current as previously described.This causes the heater 106, 206, or 306 to increase in temperature. Whenheated, the pre-tensioned tether 104, 310 will tend to recover to itsunstressed (shorter) length due to heat-induced creep. In this respect,when the tether 104, 310 is heated by the heater 106, 206, or 306; itsoverall size shrinks. However, since each end of the tether 104, 310 isfixed in place as previously described, the tether 104, 310 is unable toshorten in length, ultimately breaking to release the implant device112, 302.

Because there is tension already within the system in the form of aspring 116 or deformation of the tether material 104, 310; the amount ofshrinkage required to break the tether 104, 310 is less than that of asystem without a pre-tensioned tether. Thus, the temperature and timerequired to free the implant device 112, 302 is lower.

FIG. 5 is a graph showing the temperatures at the surface of PET cover304 of the detachment system 300. As can be seen, the surfacetemperature of the detachment system 300 during detachment does not varylinearly with time. Specifically, it only takes just under 1 second forthe heat generated by the heating coil 306 to penetrate the insulatingcover 304. After 1 second, the surface temperature of the insulatingcover 304 dramatically increases. Although different outer insulatingmaterial may slightly increase or decrease this 1-second surfacetemperature window, the necessarily small diameter of the detachmentsystem 100, 200, or 300 prevents providing a thick insulating layer thatmay more significantly delay a surface temperature increase.

It should be understood that the embodiments of the detachment system100, 200, or 300 include a variety of possible constructions. Forexample, the insulating cover 304 may be composed of Teflon, PET,polyamide, polyimide, silicone, polyurethane, PEEK, or materials withsimilar characteristics. In the embodiments 100, 200, or 300 the typicalthickness of the insulating cover is 0.0001-0.040 inches. This thicknesswill tend to increase when the device is adapted for use in, forexample, proximal malformations, and decrease when the device is adaptedfor use in more distal, tortuous locations such as, for example,cerebral aneurysms.

In order to minimize the damage and possible complications caused bysuch a surface temperature increase, the present invention detaches theimplant device 112, 302 before the surface temperature begins tosignificantly increase. Preferably, the implant device 112, 302 isdetached in less than a second, and more preferably, in less than 0.75seconds. This prevents the surface temperature from exceeding 50° C.(122° F.), and more preferably, from exceeding 42° C. (107° F.).

Once the user attempts to detach the implant device 112, 302, it isoften necessary to confirm that the detachment has been successful. Thecircuitry integrated into the power source may be used to determinewhether or not the detachment has been successful. In one embodiment ofthe present invention an initial signaling current is provided prior toapplying a detachment current (i.e. current to activate the heater 106,206, or 306 to detach an implant 112, 302). The signaling current isused to determine the inductance in the system before the user attemptsto detach the implant and therefore has a lower value than thedetachment current, so as not to cause premature detachment. After anattempted detachment, a similar signaling current is used to determine asecond inductance value that is compared to the initial inductancevalue. A substantial difference between the initial inductance and thesecond inductance value indicates that the implant 112, 302 hassuccessfully been detached, while the absence of such a differenceindicates unsuccessful detachment. In this respect, the user can easilydetermine if the implant 112, 302 has been detached, even for deliverysystems that utilize nonconductive temperature sensitive polymers toattach an implant, such as those seen in FIGS. 1, 2, and 4.

In the following description and examples, the terms “current” and“electrical current” are used in the most general sense and areunderstood to encompass alternating current (AC), direct current (DC),and radiofrequency current (RF) unless otherwise noted. The term“changing” is defined as any change in current with a frequency abovezero, including both high frequency and low frequency. When a value ismeasured, calculated and/or saved, it is understood that this may bedone either manually or by any known electronic method including, butnot limited to, an electronic circuit, semiconductor, EPROM, computerchip, computer memory such as RAM, ROM, or flash; and the like. Finally,wire windings and toroid shapes carry a broad meaning and include avariety of geometries such as circular, elliptical, spherical,quadrilateral, triangular, and trapezoidal shapes.

When a changing current passes through such objects as wire windings ora toroid, it sets up a magnetic field. As the current increases ordecreases, the magnetic field strength increase or decreases in the sameway. This fluctuation of the magnetic field causes an effect known asinductance, which tends to oppose any further change in current.Inductance (L) in a coil wound around a core is dependant on the numberof turns (N), the cross-sectional area of the core (A), the magneticpermeability of the core (p), and length of the coil (I) according toEquation 1 below:

$\begin{matrix}{L = \frac{{.4}\;\Pi\; N^{2}A_{\mu}}{l}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

The heater 106 or 306 is formed from a wound coil with proximal anddistal electrically conductive wires 108, 110, 308, or 309 attached to apower source. The tether 104, 310 has a magnetic permeability μ1 and ispositioned through the center of the resistive heater, having a lengthl, cross sectional area A, and N winds, forming a core as described inthe previous equation. Prior to detachment, a changing signaling currenti1, such as the waveforms shown in FIGS. 3A and 3B, with frequency f1,is sent through the coil windings. This signaling current is generallyinsufficient to detach the implant. Based on the signaling current, theinductive resistance XL (i.e. the electrical resistance due to theinductance within the system) is measured by an electronic circuit suchas an ohmmeter. The initial inductance of the system L1 is thencalculated according to the formula:

$\begin{matrix}{L_{1} = \frac{X_{L}}{2\;\Pi\; f_{1}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

This initial value of the inductance L1 depends on the magneticpermeability μ1 of the core of the tether 104, 310 according to Equation1, and is saved for reference. When detachment is desired, a highercurrent and/or a current with a different frequency than the signalingcurrent is applied through the resistive heater coil, causing the tether104, 310 to release the implant 112, 302 as previously described. Ifdetachment is successful, the tether 104, 310 will no longer be presentwithin the heater 106, 306 and the inside of the heater 106, 306 willfill with another material such as the patient's blood, contrast media,saline solution, or air. This material now within the heater core willhave a magnetic permeability μ2 that is different than the tether coremagnetic permeability μ1.

A second signaling current and frequency f2 is sent through the heater106, 306 and is preferably the same as the first signaling current andfrequency, although one or both may be different without affecting theoperation of the system. Based on the second signaling current, a secondinductance L2 is calculated. If the detachment was successful, thesecond inductance L2 will be different (higher or lower) than the firstinductance L1 due to the difference in the core magnetic permeabilitiesμ1 and μ2. If the detachment was unsuccessful, the inductance valuesshould remain relatively similar (with some tolerance for measurementerror). Once detachment has been confirmed by comparing the differencebetween the two inductances, an alarm or signal can be activated tocommunicate successful detachment to the user. For example, the alarmmight include a beep or an indicator light.

Preferably, the delivery system 100, 300 used according to thisinvention connects to a device that automatically measures inductance atdesired times, performs required calculations, and signals to the userwhen the implant device has detached from the delivery catheter.However, it should be understood that part or all of these steps can bemanually performed to achieve the same result.

The inductance between the attached and detached states can alsopreferably be determined without directly calculating the inductance.For example, the inductive resistance XL can be measured and comparedbefore and after detachment. In another example, the detachment can bedetermined by measuring and comparing the time constant of the system,which is the time required for the current to reach a predeterminedpercentage of its nominal value. Since the time constant depends on theinductance, a change in the time constant would similarly indicate achange in inductance.

The present invention may also include a feedback algorithm that is usedin conjunction with the detachment detection described above. Forexample, the algorithm automatically increases the detachment voltage orcurrent automatically after the prior attempt fails to detach theimplant device. This cycle of measurement, attempted detachment,measurement, and increased detachment voltage/current continues untildetachment is detected or a predetermined current or voltage limit isattained. In this respect, a low power detachment could be firstattempted, followed automatically by increased power or time untildetachment has occurred. Thus, battery life for a mechanism providingthe detachment power is increased while the average coil detachment timeis greatly reduced.

Referring now to FIG. 9, there is shown an embodiment of an implant 500of the present invention. In one embodiment, the implant 500 includes acoil 501 and a stretch-resistant member 502, the stretch-resistantmember 502 being configured to form a wrap or knot 503 about at leastone wind of said coil 501.

The coil 501 of an implant 500 may be formed from winds of, for example,metal or plastic wire. Although shown in a straight configuration inFIG. 9, a coil may also be formed into a different configuration, suchas a helical wind, flower shape, sphere, box or similar complex shape(not shown).

The stretch-resistant member 502 of an implant 500 may be formed from avariety of materials such as polyolefin, polyethylene, polyolefinelastomers such as ethylene-octene copolymer, biodegradable materialssuch as PGLA, hydrogel materials based on, for example, acrylamide orpolyethylene glycol (PEG), polyethylene terephthalate (PET), Nylon oramide-based polymers or block copolymers such as PEBAX, polypropylene,etc.

The stretch-resistant member 502 has a distal end and a proximal end andruns through the interior lumen of the coil 501 and, preferably, exitsthe winds of the coil 501 near the proximal end of the coil 501. Thestretch-resistant member 502 is then configured to wrap or tie aroundone or more winds of the coil 501 near the proximal end of the coil 501.The stretch-resistant member 502 is then preferably attached near thedistal end of a coil 501 by wrapping or tying a knot (503, as shown inFIG. 9), adhesive bonding, melting the end of the stretch-resistantmember 502 to form a ball, or a combination of the methods. In theembodiment in which a wrap or knot 503 is employed, the wrap or knot 503may be made formed from a variety of techniques including an overhandknot, a half stitch, a surgeon's knot, or a combination of wraps andknots. In a preferred embodiment, the stretch-resistant member 502 isconfigured such that it does not wrap around itself. In such aconfiguration, the tensile strength of the stretch-resistant member 502is increased.

Once a wrap or knot 503 is formed, a free (proximal) end of thestretch-resistant member 502 remains. At this point in the manufacturingprocess, the portion of the member 502 distal to the wrap or knot 503 upto the distal attachment point will provide stretch resistance to theproximal end of the member because the knot or wrap 503 (along with anyadhesive used) is configured to reduce or prevent slipping. Thus, thecoil 501 will substantially retain its original configuration as formedprior to introduction of the stretch-resistant member 502.

The proximal end of the stretch-resistant member 502 is then passedthrough, around and/or in proximity to a heater element and tension isthen placed on the proximal portion of the stretch-resistant member 502as described in U.S. patent Ser. No. 11/212,830 (as earlier describedand incorporated by reference herein). Since the distal portion of thestretch-resistant member 502 is at least partially isolated from theproximal portion of the stretch-resistant member 502 by the wrap or knot503, the tension in the distal portion of the stretch-resistant member502 that is distal to the wrap or knot 503 will be less than the tensionin the proximal portion of the stretch-resistant member 502 that isproximal to the wrap or knot 503. The proximal end or a proximal segmentof the stretch-resistant member 502 is then tied, bonded or otherwiseattached to the delivery catheter.

The implant 500 is then introduced into a body and positioned by methodsknown in the art. When the implant has been placed in the desiredposition (e.g., where the user intends to detach it from a deliverycatheter), the heating element is activated as described in U.S. patentapplication Ser. No. 11/212,830 and the member 502 is severed at or nearthe heater element. Thus, in one embodiment, the distal end of themember 502 remains bonded to the implant 500 and the proximal end of themember 502 remains bonded to the delivery catheter after the coil 501 isdetached.

As shown in FIG. 9, a preferred embodiment of the present inventionincludes a hydrogel material 504, which may be placed in proximity tothe stretch-resistant member 502. In FIG. 9, the hydrogel material runssubstantially parallel to the stretch-resistant member 502. In anotherembodiment, the hydrogel material may be wrapped, tied around orotherwise intertwined with the stretch-resistant member and/or the coil(not shown).

Although the invention has been described in terms of particularembodiments and applications, one of ordinary skill in the art, in lightof this teaching, can generate additional embodiments and modificationswithout departing from the spirit of or exceeding the scope of theclaimed invention. Accordingly, it is to be understood that the drawingsand descriptions herein are proffered by way of example to facilitatecomprehension of the invention and should not be construed to limit thescope thereof.

We claim:
 1. A detachable implant delivery system, comprising: a delivery device having a distal end and a heater proximate said distal end; a microcoil; a single monofilament attached to said microcoil at a first location on a distal region of said microcoil, at a second location on a proximal region of said microcoil, and at a third location to said delivery device proximal of said heater; wherein said single monofilament is configured to form a first wrap at said first location and a second wrap at said second location; and said first wrap and said second wrap are each formed around at least one wind of said microcoil.
 2. The detachable implant delivery system of claim 1, wherein a distal end of said single monofilament is attached to said microcoil at said first location.
 3. The detachable implant delivery system of claim 1, wherein said first location further utilizes adhesive bonding.
 4. The detachable implant delivery system of claim 1, wherein said single monofilament comprises a material selected from the group consisting of polyolefin, polyethylene, polyolefin elastomers, ethylene-octene copolymer, biodegradable materials, PGLA, hydrogel, acrylamide, PEG, PET, nylon, amide-based polymers, block copolymers, PEBAX, polypropylene.
 5. The detachable implant delivery system of claim 1, wherein said microcoil further comprises a hydrogel material.
 6. The detachable implant delivery system of claim 1, wherein said single monofilament is further connected at said first location by melting said single monofilament to form a ball.
 7. The detachable implant delivery system of claim 1, wherein said first wrap further utilizes an overhand knot, a half stitch, or a surgeon's knot.
 8. The detachable implant delivery system of claim 1, wherein the single monofilament is configured such that it does not wrap around itself.
 9. The detachable implant delivery system of claim 8, further comprising adhesive bonding at said first location on said distal region of said microcoil.
 10. The detachable implant delivery system of claim 1, wherein said second wrap is formed around a first wind of said microcoil and around a second wind of said microcoil that is adjacent to said first wind.
 11. The detachable implant delivery system of claim 1, wherein said attachment of said single monofilament at said second location isolates different tensions between a proximal portion of said single monofilament and a distal portion of said single monofilament, and so as to improve breakage upon activation of said heater and to resist stretching of said microcoil.
 12. A detachable implant delivery system, comprising: a delivery device having a heater coil at its distal end; a microcoil; and, a tether comprising a single, stretch-resistant monofilament; said tether passing through a lumen of said microcoil and a lumen of said heater coil; wherein said tether extends within said lumen of said microcoil and is attached at a first location at a distal region of said microcoil and at a second location at a proximal region of said microcoil; wherein said tether is configured to form a first wrap at said first location and a second wrap at said second location; and said first wrap and said second wrap are each formed around at least one wind of said microcoil.
 13. The detachable implant delivery system of claim 12, wherein a distal end of said tether is attached to said microcoil at said first location.
 14. The detachable implant delivery system of claim 12, wherein said tether is further attached to said microcoil at said first location with adhesive bonding.
 15. The detachable implant delivery system of claim 12, wherein said tether comprises a material selected from the group consisting of polyolefin, polyethylene, polyolefin elastomers, ethylene-octene copolymer, biodegradable materials, PGLA, hydrogel, acrylamide, PEG, PET, nylon, amide-based polymers, block copolymers, PEBAX, polypropylene.
 16. The detachable implant delivery system of claim 12, wherein said microcoil further comprises a hydrogel material.
 17. The detachable implant delivery system of claim 12, wherein said tether is further connected to said microcoil at said first location by melting said tether to form a ball.
 18. The detachable implant delivery system of claim 12, wherein the tether is configured such that it does not wrap around itself.
 19. The detachable implant delivery system of claim 18, further comprising adhesive bonding at said first location on said distal region of said microcoil.
 20. The detachable implant delivery system of claim 12, wherein said second wrap is formed around a first wind of said microcoil and a second wrap around a second wind of said microcoil that is adjacent to said first wind.
 21. The detachable implant delivery system of claim 12, wherein a distal portion of said tether has a first tension, and a proximal portion of said tether extends through a lumen of said heater coil and has a second tension, different than said first tension; said first tension configured to resist stretching of said microcoil and said second tension configured to enhance breakage upon activation of said heater coil. 